Generally, contact lenses are classified into non-water-based contact lenses and water-based contact lenses, hard contact lenses and soft contact lenses, or nonionic contact lenses and ionic contact lenses. During a long period of use of the contact lenses, microorganisms such as bacteria and fungi tend to adhere to and proliferate on the surfaces of the contact lenses while the contact lenses are stored after they have been removed from the eyes. In view of the above, contact lenses generally need to be sterilized before they are worn on the eyes. In particular, it is especially important to sterilize soft contact lenses since microorganisms are more likely to proliferate on the surfaces of soft contact lenses than on hard contact lenses, increasing the risk of causing infectious diseases.
For this reason, conventionally used liquid preparations for contact lens, in which predetermined disinfectants (preservatives) are included, for thereby executing the sterilizing treatments have been used. As the disinfectants to be added to the liquid preparations for contact lens, biguanide compounds whose typical example is polyhexamethylene biguanide (PHMB) and quaternary ammonium compounds whose typical examples are benzalkonium chloride and polyquaternium, are widely used, for instance, because these disinfectants have especially effective disinfecting properties compared to those of other disinfectants.
However, for assuring practical requirements in view of disinfection (preservation), it is necessary to increase the amount of the disinfectants to be included in the liquid preparation, even if the above-mentioned effective disinfectants are used. If the disinfectant is used in a high concentration, however, it is toxic to the eyes, and the disinfectant is easily adsorbed on the contact lens, which causes eye troubles such as eye irritation, and gives rise to a problem of insufficient safety. In view of this, various studies have been made to provide a liquid preparation for a contact lens that is capable of exhibiting a higher disinfecting effect while reducing the amount of the disinfectant that is required.
For example, JP-A-10-108899 proposed a liquid preparation for a contact lens which includes, in addition to 0.1 ppm to 10 ppm of polyhexamethylene biguanide, a nonionic isotonic agent within a sufficient ratio to provide a certain extent of an osmotic pressure, which extent is similar to that of a predetermined concentration of sodium chloride. In JP-A-11-249087, a liquid preparation for a contact lens is proposed, which includes a nonionic isotonic agent and/or an amino acid, in addition to a predetermined polyquaternium. It is true that the disinfecting effect of the liquid preparation for contact lens can be improved by adopting this constitution of the liquid preparation. However, there is an inherent problem in that, depending on the kind of nonionic isotonic agent and/or the amino acid to be used, the contact lens, especially a soft contact lens, swells or shrinks, so that the size of the contact lens is changed, whereby the specification of the contact lens is changed.
For this reason, in order to restrict changes in the size of the contact lens to be within an extent that does not substantially obstruct the wearing of the contact lens, JP-A-2001-242428 and JP-A-2002-136578 explain that it is useful to further add an amino-acid salt, in particular a sodium salt of an amino acid, to the liquid preparation for the contact lens, in addition to the above-mentioned components.
The inventors of the present invention, together with other inventors, previously proposed in JP-A-2003-160482, a liquid preparation for contact lens, which includes: a germicidal biguanide or a germicidal quaternary ammonium salt; a predetermined amino acid (salt); and a predetermined acidic compound having a carboxyl group or a phosphoric acid group, wherein the concentration of sodium chloride is 0 to 0.2% by weight. It is disclosed that, by using the liquid preparation for contact lens, the adsorption of the disinfectant on the contact lens is restricted. Due to this, safety to the eyes is assured, and swelling and shrinkage of the contact lens is prevented, whereby the problem of changes in the size of the contact lens can be advantageously solved.
However, further studies regarding the liquid preparation for a contact lens by the inventors of the present invention have newly revealed that a problem of precipitation or deposition of crystals and powdery substances on the contact lens may be caused, depending on the combination of components used. In detail, if the contact lens, especially a soft contact lens, is immersed in a liquid preparation which is obtained by combining PHMB, arginine, and glycolic acid or asparatic acid for a long period of time, or if the contact lens is repeatedly immersed in the liquid preparation, white crystals or white powdery substances, which are assumed to be derived from components of the liquid preparation, may be deposited on the surfaces of the contact lens in a dot-like form. If the components of the liquid preparation are deposited on the contact lens as described above, the field of view of the contact lens is deteriorated, which adversely affects the optical characteristics of the lens. In addition, deposits such as crystals and powdery substances cause irritation to the eyes, easily adhering the deposits to the lens, and so on. Therefore, further improvements to the liquid preparation for a contact lens are required from the point of view of preventing the deposition of components of the liquid preparation.
There can be listed the followings, as the prior art documents related to the present invention.
Patent document 1: JP-A-10-108899
Patent document 2: JP-A-11-249087
Patent document 3: JP-A-2000-242428
Patent document 4: JP-A-2002-136578
Patent document 5: JP-A-2003-160482
Patent document 6: JP-A-10-137327
Patent document 7: JP-A-11-52308
Patent document 8: JP-A-2000-513001
Patent document 9: JP-A-57-132115